This invention relates to the industrial processing of phosphoric acid, and, more particularly, to the removal of fluorine from phosphoric acid.
Phosphoric acid is widely used in commerce in the manufacture of various products, such as animal feed, food products, and fertilizer. Phosphoric acid is prepared by mining phosphate-containing ores and then producing an acid from those ores. In the "wet process", sulfuric acid is contacted to the mined ore, dissolving the phosphate values from the ore into the acid. The resulting dilute phosphoric acid is concentrated and processed to produce the required grade and purity of phosphoric acid.
In addition to the phosphate values, the sulfuric acid also dissolves other elements and compounds from the ore into the acid. These dissolved elements and compounds may be deleterious to the purity of properties of the final phosphoric acid, and therefore must be removed during the processing for certain applications. One such impurity is fluorine in the form of fluoride ion.
Phosphate-containing ore typically has a phosphorus-to-fluorine ratio of about 10. Laws regulate the amount of fluorine that may be present in animal feeds to levels judged to be safe for the animals and for consumers. Most states have laws setting the minimum phosphorus-to-fluorine ratio at 100/1. It is therefore necessary to reduce the amount of fluorine in the acid to reach the appropriate legal limit.
Several techniques are known for reducing the fluorine content of phosphoric acid. In one approach, such as described in U.S. Pat. No. 3,193,351, silica is added to the phosphoric acid. Silica reacts with various combined forms of the fluoride ion to produce silicon tetrafluoride. Silicon tetrafluoride has a high vapor pressure and is more volatile than most other combined forms of the fluoride ion. It can be driven out of the phosphoric acid during heating and concentration of the acid to merchant-grade or superphosphoric-grade acid.
Silica addition to phosphoric acid is widely used industrially to reduce the fluoride content of the acid. However, it has been observed that the use of silica additions can lead to fouling of system components such as condensers and scrubbers, or to a need for excessive filtering of the acid. There is a need for an improved approach to the production of low-fluoride-content phosphoric acid, suitable for industrial applications such as the production of animal feed. The present invention fulfills this need, and further provides related advantages.